Conventional pneumatic braking systems for heavy duty vehicles use tandem brake actuators to effect actuation of the vehicle foundation brake. These tandem brake actuators include a service brake actuator, which is responsive to fluid pressure to effect a service brake application, and also include a spring brake actuator to effect an emergency and/or parking brake application. The spring actuator includes a heavy duty spring which yieldably urges the actuator rod in a brake application effecting direction. The spring is opposed by pneumatic "hold off" pressure during normal operation of the vehicle to permit a service brake application to be effected in the normal way. However, the hold-off pressure is vented during emergency conditions or when the vehicle is parked, to permit the spring to effect an emergency or parking brake application. Both the service braking pressure and the hold-off braking pressure are controlled by pneumatic signals generated by the vehicle operator. In such conventional systems, both the service brake pressure and the hold-off pressure are routed to the tandem actuator through separate brake lines. More recently, it has been proposed to replace the conventional pneumatic braking control system with an electro-pneumatic system in which electrical signals are generated by the vehicle operator to effect control of both the vehicle service and parking/emergency braking systems. Such a system is disclosed in U.S. Pat. No. 4,749,238.